The class of metal hydrides is well known in the art, being discussed in some detail in Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd. Ed., Vol. 12, pp. 772-780. Such metal hydrides contain the element hydrogen in an electronegative state and are simple binary hydrides or are more complex. Many of the hydrides are formed by direct reaction of hydrogen and the metal or metals to be present in the hydride. In most cases, this reaction is reversible and the hydrides decompose at a more elevated temperature to regenerate elemental hydrogen. Because of these reversible reactions, a number of metals or metal alloys are useful for the storage of large volumes of hydrogen through hydride formation, which hydrogen can be released upon heating of the hydride. See, for example, the above Kirk-Othmer reference at page 779. Hydrogen-storage alloys are commercially available and have potential in solar-heating systems, nonpolluting internal combustion engines and electric utility peak-load shaving applications.
In the process of polymerizing propylene to produce polypropylene, molecular hydrogen is frequently and conventionally added to control the extent of polymerization. The proportion of hydrogen present serves to control the molecular weight of the polypropylene product which is often measured by the melt flow of the polymer expressed in dl/g. To change the melt flow and thus the molecular weight of the polypropylene product from one value to a higher value, hydrogen is typically added. To obtain polypropylene of lower melt flow a portion of the hydrogen present is removed as by reactor venting.
During ongoing commercial polypropylene production it is necessary from time to time to change the melt flow of the polymer in order to meet the specifications of polymer designed for particular applications. These changes, accomplished through modification of the proportion of molecular hydrogen present in the polymerization reactor, are costly. The lowering of melt flow typically requires the venting of hydrogen and accompanying loss of relatively large quantities of unreacted propylene monomer. In order to raise the melt flow of the polymer it is required to add hydrogen which also becomes expensive. In Burstain, U.S. Pat. No. 4,851,488, the melt flow of the polymer is reduced by consuming some of the hydrogen present through hydrogenation of unreacted monomer. Such a process cannot, of course, be used to increase polypropylene melt flow. It would be of advantage to provide a method for the control of hydrogen concentration in a polypropylene process which avoids the costs associated with the loss/replacement of hydrogen during designed changes in melt flow of a polypropylene polymer product.